KIA OPTIMA ENGINE PROBLEM

2001 Kia Optima • 12,000 miles

The car is a2.4 litre 16 valve. First does it come with two catlitic converters. The first one has the oxygen sensers 1upstream and one down stream then there is a second catlitic conv. About 3 feet down stream with no sensers. The lady had the second one changed about a year ago. She complained she has no power. I drove the car and wouldnt get to twenty miles an hour. The engine wouldnt reve past 3000 rpm. Well I removed the front cat because the codes were p0300 and p0421. The converter was empty so I cut out the second one and shook it out and got maybe a pound of stuff I think about half of what I should have gotten. Is the second one clogged agin or could it be the muffler because its had two cats fail on it in one year. Or did the first one fail because the front one started comming apart over a year ago and it ruiened a second cat. Agin. When I shook it out it was better but far from right. Also could it be the sensers now causing the lack of power.. Thank you I would appreciate a second or even third opinion.I am doimg this job pro bono so I wont be able to donate the lady lives in her car and if she gets stranded they will tow her home.